CFStringRef to NSString ARC leaking. Why?

问题:

I have been looking around for the correct way to go from CFStringRef to NSString in ARC to avoid memory leaks and some of the main voted answers suggest:

NSString * string = (__bridge NSString *)cfString;

I am using that approach here but when profiling the app I still get a memory leak in this small method [see attached image].

So, I am not sure how to fix this. Anybody has the solution to this problem?

Thank you

So, apparently adding the CFRelease(ext) before the return fixed the leak. Problem is I don't think I fully understand the reason. I thought that the line:

NSString * extension = (__bridge NSString*)ext

Would take ownership of the Core Foundation ext string and handle the release. Can anybody explain what is going on here exactly?

回答1:

As per the de-facto "standard" Cocoa naming convention, functions that contain Create or Copy in their name return an object with a reference count of 1. You have to transfer this reference count into ARC-land so that ARC can take care of it. You use the __bridge_transfer keyword for this purpose.

NSString *string = (__bridge_transfer NSString *)cfString;

回答2:

Release the ext object or use __bridge_transfer.

Let me explain this a bit in the way I understand it:

• __bridge – Normal casting. The retain count of casted object is partially managed by ARC and partially manually. You need to release the existing ownership on the CF side.

• __bridge_transfer – “Converts CF object to NS object.” The retain count of casted object is fully managed by ARC. Existing ownership on the CF side is disposed for you.